Process for preparing calcium sulfate alpha hemihydrate

ABSTRACT

In a process for preparing calcium sulphate alpha hemihydrate essentially  the form of primary crystals, the hemihydrate is milled to specific surface area of 1200 to 4000 cm 2  /g, the fines are removed by screening, and a retarder and liquefier in the form of at least one fruit acid and/or its or their salt(s) are admixed. The product obtained is characterized by delayed onset of rigidity, rapid development of resistance and high resistance, and is used in particular as an injection mortar.

FIELD OF THE INVENTION

The invention relates to a process for the preparation of calciumsulfate alpha hemihydrate.

THE RELATED ART

Various processes are known for the preparation of calcium sulfate alphahemihydrate, namely so-called wet processes, wherein raw gypsum iscontinuously passed in a suspension through an autoclave and convertedto calcium sulfate hemihydrate, as well as autoclave methods wherein theraw gypsum is introduced in pieces or as briquettes in an autoclave andconverted. For instance, in the preparation of calcium sulfate alphahemihydrate from raw gypsum originating from desulfurizationinstallations of power plants and which has been pressed intobriquettes, it is possible to obtain large and regularly shaped crystals(primary crystals) of calcium sulfate alpha hemihydrate, after theautoclave process. The briquettes consisting of primary crystals arebroken in a crusher, and this way the primary crystal is broken downinto a coarse-grained product with a specific surface of for instance800 cm² /g.

Such coarse-grained calcium sulfate alpha hemihydrate is in itself avery valuable product when used for the preparation of mortars or thelike, due to its reduced water requirements. However, the coarse graindoes not lead to a rapid strength development, since the reaction pertime unit of the coarse grain with water is relatively reduced andincomplete. In order to accelerate this reaction, the usual acceleratingagents can be used. However, they have the disadvantage of leadingimmediately to onset of strength development. Although this effect canbe diminished by feeding additional water amounts, these additionalwater amounts are actually undesirable. Besides, usually a sufficientholding time is required, in order to prepare the mortar and to bring itto the place where it is needed.

It is therefore the object of the present invention to create a processof the aforementioned kind, which renders possible production of acalcium sulfate alpha hemihydrate with a sufficiently delayed onset ofrigidity, but which still leads to a rapid strenth development and ahigh level of strength.

SUMMARY OF THE INVENTION

This problem is solved by bringing the calcium sulfate alphahemihydrate, which basically is available in primary crystals, to adegree of fineness corresponding to the strength development requiredfor the intended use, by removing fines through screening and byadmixing at least one of the fruit acids and/or their salts asretardants or liquefiers.

By milling the initial coarse-grained product, specific surfaces in therange of approximately 1200 to 4000 cm² /g, particularly 1500 to 3500cm² /g are selected. Due to the screening, a certain range of grain sizecan be selected, so that the finest grain, as well as the coarser graincan be eliminated. The fine grain is extremely reactive and has a highwater requirement, so that thereby the desirable characteristics wouldbe impaired. Coarser grain does not have a high water requirement, butis not sufficiently reactive.

The fruit acids, e.g. malic acid and tartaric acid, particularly citricacid, or their water-soluble salts, which can be used separately or incombination, delay the onset of rigidity. At the same time thesecompounds serve as liquefiers, whose effect starts immediately when thecalcium sulfate alpha hemihydrate comes in contact with the mixingwater. Besides, as a result, the compression strength is considerablyincreased. This is particularly surprising, since up to now citric acidhas been considered as an additive with a delaying, strength-reducingeffect, when used in small amounts under 0.1M. -%, while in largeramounts it even avoids strength development, compare with ULIMANNSENZYKLOPADIE DER TECHNISCHEN CHEMIE, 4th Edition, Volume 12, Page 307,and R. A. Kuntze, THE CHEMISTRY AND TECHNOLOGY OF GYPSUM, ASTM SpecialTechnical Publication 861, 1984, so that citric acid, as well as otherrelated fruit acids were considered up to now as undesirable.

The fruit acid is added particularly in an amount of 0.005 to 0.05M.-%,preferably in an amount of 0.02M.-%, possibly as citric acidmonohydrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between the onset of rigidity inrelationship to the grain fineness (surface area) of the calcium sulfatealpha hemihydrate.

FIG. 2 shows the relationship between strength and the grain fineness(surface area) of the calcium sulfate alpha hemihydrate.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the rigidity of calcium sulfate alpha hemihydrate isdiagrammatically shown in relation to the grain fineness. Needledistance in mm of a Vicat apparatus based on DIN 1168 is shown on theordinate. Time is shown in minutes on the abscissa. It is evident thatonset of rigidity is pushed towards shorter times with the increase inthe specific surface O_(sp). By adding 0.02M.-% citric acid monohydratethe onset of rigidity in a paste made of calcium sulfate alphahemihydrate with a specific surface of 3000 cm² /g was pushed up fromapproximately 7 minutes to approximately 30 minutes.

As can be seen from FIG. 2, the milling fineness influences the strengthdevelopment. In FIG. 2, the compression strength in M/mm² is entered onthe ordinate, while the time in hours is entered on the abscissa,whereby the various curves A to E represent the strength development inpastes of calcium sulfate hemihydrate with the specific surfaceindicated each time, in the case of an addition of 0.02M.-% of citricacid monohydrate.

As can be seen from the figure, the finest grain (specific surface 4700cm² /g, grain size 99% under 0.03 mm) leads to a decrease of thecompression strength in comparison with a binder granulation of 3000 or3500 cm² /g. The water requirement, expressed in the ratio water/gypsumequals 0.28 in each of these cases.

Besides, for comparison, FIG. 2 shows the strength development for apaste with a specific surface of 700-900 cm² /g (water/gypsum ratio of0.31), as well as 3500 cm² /g (water/gypsum ratio 0.35), compare curvesF,G, but without the addition of fruit acids. Here, the achievedstrength levels are considerably below the ones achieved with theaddition of fruit acids. Besides, it becomes evident that thecomminution of coarse-grained material, in the absence of fruit-acidadditives, has a negative influence on the strength development.

By using calcium sulfate alpha hemihydrate with various fineness, thestrength development can be correspondingly controlled.

Preferably, the fruit acids and/or their salts are admixed to the milledand screened calcium sulfate alpha hemihydrate as fine particles, but itis also possible to dissolve them in the mixing water and this way toreach the desired strength development and the increased strength level.Besides, the salts of the fruit acid can be introduced into the productalready in a previous preparation process, in that the fruit acid or itssalt, has already been added as an agent influencing crystalline growthin the recrystallization of calcium sulfate dihydrate to alphahemihydrate. The process is particularly suited for the use of alphahemihydrate which has been prepared through recrystallization ofdihydrate in autoclaves with the addition of a crystalline growthinfluencing agent in the form of finely ground brown coal and/or peatand or finely ground wood and/or humic acid and or wood-components withthe same effect, preferably with a particle size under 100 μm. This canbe ground brown coal, in an amount of 0.1 to 1.0% by mass, preferably0.5 to 0.7% by mass. It is possible to use ground peat in an amount of0.1 to 1.5% by mass, preferably 0.5 to 1% by mass. Wood components withsimilar effects are also ground wood, which is added in an amount of 0.3to 2.0% by mass preferably 0.7 to 1.5% by mass. Humic acid can be addedin an amount of 0.1 to 1.0% by mass, preferably 0.3 to 7% by mass aswood-component. Sulfite waste liquor functions as a growth-influencingadditive in an amount of 0.1 to 3.0% by mass, preferably 0.5 to 2% bymass. Also components of sulfite waste liquor having the same effect orsecondary conversion products of native lignine such as for instancelignine-sulfonic acid, can be used in an amount of 0.1 to 1.5% by mass,preferably 0.3 to 0.8% by mass. Finally, these additives can also belignine sulfonate in an amount of 0.1 to 1.2% by mass, preferably of 0.3to 0.8% by mass. A further growth influencing step consists in that apart of the calcium sulfate alpha hemihydrate to be converted originatesfrom the flue-gas desulfurization gypsum of a power plant run with browncoal, for instance in an amount of at least 25% by mass.

The calcium sulfate alpha hemihydrate prepared this way is particularlysuited for the production of pneumatically transportable injectionmortars, which are wetted with water at discharge, for dam constructionmaterials affording immediate- or preliminary support for undergrounduse.

I claim:
 1. A process for preparing calcium sulfate alpha hemihydrate ingranular form having a specific surface area of 1200 to 4000 cm² /g,which comprises the steps of:(a) milling crystalline calcium sulfatealpha hemihydrate to obtain a granular calcium sulfate alpha hemihydratehaving a specific surface area of 1200 to 4000 cm² /g; (b) screening thegranular calcium sulfate alpha hemihydrate produced during step (a) toeliminate coarser and finer grains, after said screening said producthaving a specific surface area of 1200 to 4000 cm² /g and particle sizegreater than 0.03 mm; and (c) admixing 0.005 to 0.05% of at least onefruit acid selected from the group consisting of malic acid, tartaricacid, citric acid, and their salts and monohydrates, with the ground andscreened calcium sulfate alpha hemihydrate produced during steps (a) and(b).
 2. Process according to claim 1, wherein the screening stepachieves a hemihydrate having a specific surface approximately between1500 and 3500 cm² /g.
 3. Process according to claim 1, wherein the fruitacids are admixed in an amount of from 0.005 to 0.02% by mass. 4.Process according to claim 1, wherein the fruit acid is citric acidmonohydrate.
 5. Process according to claim 1, wherein the fruit acidsare admixed as fine particles.
 6. Process according to claim 1, furthercomprising adding to the milled calcium sulfate alpha hemihydrate agrowth-influencing additive selected from the group consisting of finelyground brown coal, finely ground peat, finely ground wood, humic acid,and wood components.
 7. Process according to claim 1, wherein thecrystalline calcium sulfate alpha hemihydrate has been produced fromcalcium sulfate dihydrate from a flue-gas desulfurization installationof a power plant running on brown coal.